For the offshore site of Hebron oil field in Canada, ExxonMobil opted to build a platform installed on a separate Gravity Based Structure (GBS). The platform is positioned in the Atlantic Ocean at a depth of 93 metres, 350 km off the Canadian coast of Newfoundland.
The imposing concrete structure is designed to store up to 1.2 million barrels of crude oil.
Main contractorKiewit-Kværner Contractors (KKC)
PartnersKKC EPC Contractor
DNV Offshore Certification
An offshore platform with huge dimensions
The gravity based structure (GBS) is 130 m in diameter and 20 m high. The full structure is 93 meters high. It has seven storage compartments, surrounded by seven ice walls to protect against icebergs.
The height of the structure and the choice of U-shaped tendons for the vertical prestressing led to the use of particularly long tendons: they are 140 m long in the storage and external cells, and 234 m long in the central column. The duct injection method was also formulated to ensure a perfect filling of the duct, without any voids.
Consisting of 19, 22, and 31 strands each (and anchorage with the Freyssinet C-Range anchorage system), the tendons were prefabricated onshore before being transported to the GBS by means of an equipment specially designed and manufactured by Freyssinet.
Onshore and offshore prestressing operations
The first phase of the operations, up to elevation 27.5 m, took place in a dry dock. Then the Gravity Based Structure was towed offshore to complete its construction, up to elevation 72 m for the storage cells and 125 m for the shaft.
This operation was executed using nine towing points fixed to the structure by eight harnesses of Freyssinet tendons (12C15 anchorages). The mooring anchors were also secured to the structure using prestressing tendons.
“Close coordination and scheduling with KKC was necessary to carry out the work whenever the weather permitted, roughly from April to November. Most tensioning work had to be carried out in the last construction phase over a narrow time window starting in spring 2016, before the deadline for the lifting of the topsides and the next winter period. 1,500t of prestressing remained to be installed by mid-September, out of a total of 2,400t.” explains Rodolphe Degonse, Former manager of Nuclear Oil & Gas Business Unit of Freyssinet.
Strong technical constraints
The second phase of the post-tensioning work performed at sea faced a major technical event: Sea water infiltration throughout the concrete. Freyssinet and KKC developed work procedure, and technical solution to:
- Remove corrosive agent from the post tensioning duct before installation of the tendon,
- Ensure sealing of the seawater infiltration sources to maintain high sustainability standard on the post tensioning work (and maintain the DNV certification of the completed structure).,
- And achieve a high-quality level of grout filling of the tendons (assessed by mock up and vents inspections).
“Technical studies and trials were performed to allow the grout injection under colder temperatures than usually authorized by standards. In addition, water Grout injection mock-ups were prepared and tested ahead of the last installation phase, in order to secure the start of the post-tensioning works as soon as the weather would permit and allow flawless operations.” explains Rodolphe Degonse, Former manager of Nuclear Oil & Gas Business Unit of Freyssinet.
Final sprint for the offshore platform
Once the GBS has been completed and the top side installed using Freyssinet tensioning bars, the structure was towed to its final location using around 10 tugs: 7 to apply a towing force of 1,350 t and 3 to apply a reverse force of 500 t. The tugs, equipped with four anchors weighing 15 to 20 t. each, were also used to install and hold the GBS in position.
Lastly, the GBS was ballasted to rest on the seabed, before being connected to the well heads.
1,2MStorage capacity in number of oil barrels (millions)
130mDiameter of the structure base
93mHeight of the structure
2,400tTotal weight of post-tensioning installed (tonnes)
234mMaximum length of the prestressing tendons